CD8+ T lymphocytes play an important role in the control of human immunodeficiency virus type 1 (HIV) infection via HIV-specific cytotoxic T lymphocyte (CTL) activity and the release of soluble factors. We have obtained evidence that CD8+ T cells are killed by macrophages (Mphi) during HIV infection (47) (MS#1). Following interaction of CXCR4 with the HIV envelope gp120, tumor necrosis factor receptor 2 (TNFR2) is up-regulated on Cd8+ T cells and tumor necrosis factor alpha (TNF) stimulation provided by Mphi triggers CD8+ T cell death. In our preliminary data we observed that CD8+ T cells undergoing apoptosis in the presence of Mphi express highlevels of Cd8 and TNFR2 on the cell surface, but low levels of CD28. We also observed that CXCR4-mediatd TNFR2 induction occurs in CD8+ T cells but not in CD4+ T cells. We hypothesize that CD8+ T cells killed by Mphi contain HIV-specific effector cells, which can be prevented from death using small-molecule inhibitors that block the chemokine receptor CXCR4, and used for adoptive transfer. We also hypothesize that Mphi-mediated CD8+ T cell apoptosis is triggered via TNFR2 pathway following CXCR4 stimulation, and that low levels of CD28 do not allow rescue of CD8+ T cells from death. The specific aims are as follows: Aim 1. To determine whether CD8+ T cells undergoing apoptosis during HIV infection have antiviral function,. we will assess whether CD28CD8+ T cell subsets contain HIV-specific effector cells. Therefore, we will measure cytotoxic T lymphocyte (CTL) activity within CD28CD8+ T cell subsets by using class I tetrameric labeled proteins containing specific HLA-restricted peptides, by direct cytolysis assays, and by limiting dilution analysis (LDA) assay. Also we will measure the production of cytokines by CD28CD8+ T cell subsets, such as beta-chemokines, TH1 cytokines, and TH2 cytokines. Aim 2. To determine whether CD8+ T cells from peripheral blood of HIV-infected subjects can be spared and/or expanded, we will utilize small-molecules directed against CXCR4 to try to prevent CD8+ T cell death. We will also test the proliferative capacity of CD28CD8+ T cell subsets and perform a clonal expansion of CD28CD8+ T cell subsets in vitro that will be tested for their HIV-specific CTL activity. Aim 3. To determine the molecular mechanisms underlying CD8+ T cell apoptosis in HIV infection, we will first access the involvement of the CXCR4 chemokine receptor in CD8+ T cell apoptosis. We will then characterize the pathways downstream of TNFR2 that are involved in CD8+ T cell apoptosis. Finally, we will assess whether the level of CD28 expression affects the recovery of CD8+ T cells from apoptosis.